Codeine is also commonly marketed in products containing codeine with other pain killers or muscle relaxers, as well as codeine mixed with phenacetin (Emprazil With Codeine No. 1, 2, 3, 4 and 5), naproxen, indomethacin, diclofenac, and others, as well as more complex mixtures, including such mixtures as aspirin + paracetamol + codeine ± caffeine ± antihistamines and other agents, such as those mentioned above.

Codeine-only products can be obtained with a prescription as a time release tablet (e.g., Codeine Contin 100 mg and Perduretas 50 mg). Codeine is also marketed in cough syrups with zero to a half-dozen other active ingredients, and a linctus (e.g., Paveral) for all of the uses for which codeine is indicated.

Injectable codeine is available for subcutaneous or intramuscular injection only; intravenous injection is contraindicated as this can result in non-immune mast-cell degranulation and resulting anaphylactoid reaction. Codeine suppositories are also marketed in some countries.

Codeine and morphine as well as opium were used for control of diabetes until relatively recently,[9] and still are in rare cases in some countries, and the hypoglycemic effect of codeine, although usually weaker than that of morphine, diamorphine, or hydromorphone, can lead to cravings for sugar.

Tolerance to many of the effects of codeine develops with prolonged use, including therapeutic effects. The rate at which this occurs develops at different rates for different effects, with tolerance to the constipation-inducing effects developing particularly slowly for instance.

A potentially serious adverse drug reaction, as with other opioids, is respiratory depression. This depression is dose-related and is a mechanism for the potentially fatal consequences of overdose. As codeine is metabolized to morphine, morphine can be passed through breast milk in potentially lethal amounts, fatally depressing the respiration of a breastfed baby.[10][11] In August 2012, the Federal Drug Administration issued a warning about deaths in pediatric patients < 6 years old after ingesting "normal" doses of paracetamol with codeine after tonsillectomy.[12]

Some patients are very effective converters of codeine to its active form, morphine, resulting in lethal blood levels. The FDA presently is recommending very cautious use of Codeine in young tonsillectomy patients: use the drug in the lowest amount that can control the pain, use "as needed" and not "around the clock", and seek immediate medical attention if a child on codeine exhibits excessive sedation or abnormally noisy breathing.

Codeine is metabolized to codeine-6-glucuronide (C6G) by uridine diphosphate glucuronosyl transferase UGT2B7, and, since only about 5% of codeine is metabolized by cytochrome P450 CYP2D6, the current evidence is that C6G is the primary active compound.[14] Claims about the supposed "ceiling effect" of codeine doses are based on the assumption that high doses of codeine saturate CYP2D6, preventing further conversion of codeine to morphine, however it is now known that C6G is the main metabolite responsible for codeine's analgesia.[15]

There is also no evidence that CYP2D6 inhibition is useful in treating codeine dependence,[16] though the metabolism of codeine to morphine (and hence further metabolism to glucuronide morphine conjugates) does have an effect on the abuse potential of codeine.[17] However, CYP2D6 has been implicated in the toxicity and death of neonates when codeine is administered to lactating mothers, particularly those with increased 2D6 activity ("ultra-rapid" metabolizers).[18][19]

The conversion of codeine to morphine occurs in the liver and is catalyzed by the cytochrome P450 enzyme CYP2D6. CYP3A4 produces norcodeine and UGT2B7 conjugates codeine, norcodeine, and morphine to the corresponding 3- and 6- glucuronides. Srinivasan, Wielbo and Tebbett speculate that codeine-6-glucuronide is responsible for a large percentage of the analgesia of codeine, and, thus, these patients should experience some analgesia.[20] Many of the adverse effects will still be experienced in poor metabolizers. Conversely, 0.5-2% of the population are "extensive metabolizers"; multiple copies of the gene for 2D6 produce high levels of CYP2D6 and will metabolize drugs through that pathway more quickly than others.

CYP2D6 converts codeine into morphine, which then undergoes glucuronidation. Life-threatening intoxication, including respiratory depression requiring intubation, can develop over a matter of days in patients who have multiple functional alleles of CYP2D6, resulting in ultra-rapid metabolism of opioids such as codeine into morphine.[21][22][23]

Studies on codeine's analgesic effect are consistent with the idea that metabolism by CYP2D6 to morphine is important, but some studies show no major differences between those who are poor metabolizers and extensive metabolizers. Evidence supporting the hypothesis that ultrarapid metabolizers may get greater analgesia from codeine due to increased morphine formation is limited to case reports.[24]

Due to increased metabolism of codeine to morphine, ultrarapid metabolizers (those possessing more than 2 functional copies of the CYP2D6 allele) are at increased risk of adverse drug effects related to morphine toxicity. Guidelines released by the Clinical Pharmacogenomics Implementation Consortium (CPIC) advise against administering codeine to ultrarapid metabolizers, where this genetic information is available. The CPIC also suggests that codeine use be avoided in poor metabolizers, due to its lack of efficacy in this group.[25]

Codeine and its salts are readily absorbed from the gastrointestinal tract and ingestion of codeine phosphate produces peak plasma concentrations in about one hour. Plasma half life is between 3 to 4 hours and oral/intramuscular analgesic potency ratio is approximately equal to 1:1.5. It is metabolised by O- and N-demethylation in the liver to morphine and norcodeine. Codeine and its metabolites are excreted almost entirely by the kidney, mainly as conjugates with glucuronic acid.[26]

The active metabolites of codeine, notably morphine, exert their effects by binding to and activating the μ-opioid receptor.

Codeine has been used in the past as the starting material and prototype of a large class of mainly mild to moderately strong opioids; such as hydrocodone (1920 in Germany), oxycodone (1916 in Germany), dihydrocodeine (1908 in Germany), and its derivatives such as nicocodeine (1956 in Austria).[citation needed] However, these opiates are no longer synthesized from codeine and are usually synthesized from other opioid alkaloids; specifically thebaine.[citation needed] Other series of codeine derivatives include isocodeine and its derivatives, which were developed in Germany starting around 1920.

As an analgesic, codeine compares moderately to other opiates. Related to codeine in other ways are codoxime, thebacon, codeine-N-oxide (genocodeine), related to the nitrogen morphine derivatives as is codeine methobromide, and heterocodeine, which is a drug six times stronger than morphine and 72 times stronger than codeine due to a small re-arrangement of the molecule, viz. moving the methyl group from the 3 to the 6 position on the morphine carbon skeleton.

Drugs bearing resemblance to codeine in effects due to close structural relationship are variations on the methyl groups at the 3 position including ethylmorphine a.k.a. codethyline (Dionine) and benzylmorphine (Peronine). While having no narcotic effects of its own, the important opioid precursor thebaine differs from codeine only slightly in structure. Pseudocodeine and some other similar alkaloids not currently used in medicine are found in trace amounts in opium as well.

Codeine, or 3-methylmorphine, is an alkaloid found in the opium poppy, Papaver somniferum var. album, a plant in the papaveraceae family. Opium poppy has been cultivated and utilized throughout human history for a variety of medicinal (analgesic, anti-tussive and anti-diarrheal) and hypnotic properties linked to the diversity of its active components, which include morphine, codeine and papaverine.

Codeine is found in concentrations of 1.0 to 3.0 per cent in opium prepared by the latex method from unripe pods of Papaver somniferum. The name codeine is derived from the Greek word kodeia (κώδεια) for "poppy head". The relative proportion of codeine to morphine, the most common opium alkaloid at 4 to 23 per cent, tends to be somewhat higher in the poppy straw method of preparing opium alkaloids.

Until the beginning of the 19th century, raw opium was used in diverse preparations known as laudanum (see Thomas de Quincey's "Confessions of an English Opium-Eater", 1821) and paregoricelixirs, a number of which were popular in England since the beginning of the 18th century; the original preparation seems to have been elaborated in Leiden, the Netherlands around 1715 by a chemist named Lemort; in 1721 the London Pharmocopeia mentions an Elixir Asthmaticum, replaced by the term Elixir Paregoricum ("pain soother") in 1746.

The progressive isolation of opium's several active components opened the path to improved selectivity and safety of the opiates-based pharmacopeia.

Morphine had already been isolated in Germany by German pharmacist Friedrich Sertürner in 1804.[27] Codeine was first isolated decades later in 1832 in France by Pierre Robiquet, a French chemist and pharmacist already famous for the discovery of alizarin, the most widespread red dye, while working on refined morphine extraction processes. This paved the way for the elaboration of a new generation of safer, codeine-based specific antitussive and antidiarrheal formulations.

Codeine is currently the most widely used opiate in the world,[28][29] and is one of the most commonly used drugs overall according to numerous reports by organizations including the World Health Organization and its League of Nations predecessor agency. It is one of the most effective orally administered opioid analgesics and has a wide safety margin. Its strength ranges from 8 to 12 percent of morphine in most people; differences in metabolism can change this figure as can other medications, depending on its route of administration.

While codeine can be directly extracted from opium, its original source, most codeine is synthesized from the much more abundant morphine through the process of O-methylation.[29][30]

By 1972, the effects of the War On Drugs had caused across-the-board shortages of illicit and licit opiates because of a scarcity of natural opium, poppy straw, and other sources of opium alkaloids, and the geopolitical situation was growing difficult for the United States. After a large percentage of the opium and morphine in the US National Stockpile of Strategic & Critical Materials was tapped in order to ease severe shortages of medicinal opiates — the codeine-based antitussives in particular — in late 1973, researchers were tasked with finding a way to synthesize codeine and its derivatives. They quickly succeeded using petroleum or coal tar and a process developed at the United States' National Institutes of Health.

Numerous codeine salts have been prepared since the drug was discovered. The most commonly used are the hydrochloride (freebase conversion ratio 0.805), phosphate (0.736), sulphate (0.859), and citrate (0.842). Others include a salicylateNSAID, codeine salicylate (0.686), and at least four codeine-based barbiturates, the cyclohexenylethylbarbiturate (0.559), cyclopentenylallylbarbiturate (0.561), diallylbarbiturate (0.561), and diethylbarbiturate (0.619).

In some countries, cough syrups and tablets containing codeine are available without prescription; some potential recreational users are reported to buy codeine from multiple pharmacies so as not to arouse suspicion. In some countries, in an effort to reduce recreational use, all OTC purchases of codeine are electronically recorded, and any pharmacy can access these records if desired. A heroin addict may use codeine to ward off the effects of a withdrawal.[31]

Codeine is also available in conjunction with the anti-nausea medication promethazine in the form of a syrup. Brand named as Phenergan with Codeine or in generic form as promethazine with codeine. Called 'syrup', 'lean', or 'purple drank', this medication is quickly becoming one of the most commonly misused codeine preparations.[32] Rapper Pimp C, from the group UGK, died from an overdose of this combination.[33] This mixture is, owing to promethazine being a prescription-only antihistamine, available only on prescription in all 50 USA states.

While all sedating antihistamines will potentiate the effects of codeine, enzyme induction leading to greater hepatic conversion of codeine to morphine is the reason for both the clinical and recreational use of promethazine with codeine. This was the case all the more with another drug, Doriden (glutethemide) (q.v.) Codeine and glutethimide were taken in combination known as Loads, Dors & Fours, and Six Packs (two Doriden tablets or 1000 mg of glutethimide and four tablets of a codeine combination totalling 240 mg of codeine) as a substitute for heroin or a primary opioid drug of choice; first mentioned in medical journals in the late 1960s, then reappearing along with Ts & Blues (pentazocine and tripelennamine) during the late 1970s. Glutethimide was off the market by the end of 1988 around the world, but the use of this combination is apparently a major driver for the small amount of illicit glutethimide synthesis uncovered since that time. Taking the combination on an empty stomach was particularly favoured for partially approximating the rush and bang of injecting heroin or morphine.[34]

Codeine is also demethylated by reaction with pyridine to synthesize morphine, which can then be acetylated to make heroin (diacetylmorphine). Pyridine is toxic and possibly carcinogenic, so morphine produced in this manner (and potentially contaminated with pyridine) may be particularly harmful.[35] Codeine can also be turned into α-chlorocodide, which is used in the clandestine synthesis of desomorphine (Permonid) (desomorphine attracted attention in 2010 in Russia due to an upsurge in clandestine production, presumably due to its relatively simple synthesis from codeine.[citation needed] The drug is easily made from codeine, iodine and red phosphorus,[36] in a similar process to the manufacture of methamphetamine from pseudoephedrine, but desomorphine made this way is highly impure and contaminated with various toxic and corrosive byproducts.).

Codeine and/or its major metabolites may be quantitative in blood, plasma or urine to monitor therapy, confirm a diagnosis of poisoning or assist in a medico-legal death investigation. Drug abuse screening programs generally test urine, hair, sweat or saliva. Many commercial opiate screening tests directed at morphine cross-react appreciably with codeine and its metabolites, but chromatographic techniques can easily distinguish codeine from other opiates and opioids. It is important to note that codeine usage results in significant amounts of morphine as an excretion product. Furthermore, heroin contains codeine (or acetyl codeine) as an impurity and its use will result in excretion of small amounts of codeine. Poppy seed foods represent yet another source of low levels of codeine in one's biofluids. Blood or plasma codeine concentrations are typically in the 50–300 µg/L range in persons taking the drug therapeutically, 700–7000 µg/L in chronic users and 1000–10,000 µg/L in cases of acute fatal over dosage.[37][38][39]

In Australia, codeine preparations must be sold only at a pharmacy. Preparations will often be a combination of paracetamol (500 mg), ibuprofen (200 mg) and doxylamine succinate (5 mg), and the codeine content may range from 5 mg to 15 mg; preparations with in excess of 15 mg per tablet are S4 (Schedule 4, or "Prescription Only") medications. The item is given over the counter, no prescriptions, at the discretion of the pharmacist. Most preparations are considered S3 (schedule 3, or "Pharmacist Only") medications, meaning that they must be sold with the direct involvement of a pharmacist. It must be labelled and usage history monitored by the pharmacist to help deter misuse and dependence. The exception to this rule is cold and flu preparations such as "Codral". These preparations contain phenylephrine (5 mg), paracetamol (500 mg) and codeine (9.5 mg).[40]

Preparations containing pure codeine (e.g., codeine phosphate tablets or codeine phosphate linctus) are available on prescription and are considered S8 (Schedule 8, or "Controlled Drug Possession without authority illegal"). Schedule 8 preparations are subject to the strictest regulation of all medications available to consumers.

In Canada tablets containing 8mg of codeine combined with 15mg of caffeine and 300mg of paracetamol are sold as T1s (Tylenol Number 1) without a prescription. A similar tablet called an AC&C which contains 325mg of acetylsalicylic acid (Aspirin) instead of paracetamol is also available without a prescription. Both tablets are kept behind the counter and must be dispensed by a pharmacist who may limit quantities.

In Denmark codeine is sold over the counter with max 9.6 mg in mixture.[citation needed] The item is given over the counter, no prescriptions. The strongest available over the counter preparation containing codeine has 9.6 mg (with aspirin, brand name Kodimagnyl); anything stronger requires a prescription.

Codeine is listed under the Betäubungsmittelgesetz in Germany and the similarly named narcotics and controlled substances law in Switzerland. In Austria, the drug is listed under the Suchtmittelgesetz in categories corresponding to their classification under the Single Convention on Narcotic Drugs. Dispensing of products containing codeine and similar drugs (dihydrocodeine, nicocodeine, benzylmorphine, ethylmorphine etc.), in general, requires a prescription order from a doctor or the discretion of the pharmacist.

Municipal and provincial regulations may impact availability, in particular in Austria and Switzerland, which allows cities and provinces to regulate the selling of the least-regulated schedule of the SMG/BtMG. Individual chemists' shops can opt out of providing them or imposing volume, frequency, or single-purchase limitations and other things of the same store. Plain codeine hydrochloride tablets (which in the USA would share CSA Schedule II with drugs like morphine, cocaine, hydromorphone, and bezitramide) as well as other non-injectable forms of codeine and its midrange derivatives can be dispensed in this way; the same goes for most chemical classes of benzodiazepines, the majority of non-barbiturate sedative/hypnotics, and at least a handful of barbiturates.

Title 76 of the Schengen treaty has made it possible for countries within the signatory states to import and export drugs with various provisos, recording and ordering requirements, and various other rules.

Codeine is classed as an illegal drug in Greece, and individuals possessing it could conceivably be arrested, even if they were legitimately prescribed it in another country. It is sold only with a doctor's prescription (Lonarid-N, Lonalgal).[42]

In Hong Kong, codeine is regulated under Laws of Hong Kong, Dangerous Drugs Ordinance, Chapter 134, Schedule 1. It can be used legally only by health professionals and for university research purposes. The substance can be given by pharmacists under a prescription. Anyone who supplies the substance without prescription can be fined $10,000 (HKD). The maximum penalty for trafficking or manufacturing the substance is a $5,000,000 (HKD) fine and life imprisonment. Possession of the substance for consumption without license from the Department of Health is illegal with a $1,000,000 (HKD) fine and/or 7 years of jail time.

However, codeine is available without prescription from licensed pharmacists in doses up to 0.1% [43]:Schedule 1, Part IV, paragraph 23 (i.e. 5 mg/5ml)[43]:Section 3, (1) (a)

All pharmaceuticals containing codeine in Iceland require a prescription from a doctor. 10mg of codeine against 500mg of paracetamol are sold under the name of Parkódín, and 30mg of codeine against 500mg of paracetamol are sold under the name of Parkódín Forte.

Codeine preparations require a prescription in India. A preparation of paracetamol and codeine is available in India. Codeine is also present in various cough syrups as codeine phosphate including chlorpheniramine maleate. Pure codeine is also available as codeine sulphate 15mg tablets.

Preparations of codeine in Iran normally comes with paracetamol, but can be purchased over-the-counter. Iran's deputy health minister reported that codeine is Iran's best selling OTC medication. The recreational use of codeine has also become widespread throughout Iran but authorities continue to let codeine be purchased without permission from a doctor, although the pharmacist may ask for the identification of the purchaser to verify they are 18 years or older to buy.

In Ireland, new regulations came into effect on August 1, 2010[44] concerning codeine, due to worries about the overuse of the drug. Codeine remains a semi non-prescriptive, over-the-counter drug up to a limit of 12.8 mg per pill, but codeine products must be out of the view of the public to facilitate the legislative requirement that these products “are not accessible to the public for self-selection”.[45] In practice, this means customers must ask pharmacists for the product containing codeine in name, and the pharmacist makes a judgement whether it is suitable for the patient to be using codeine, and that patients are fully advised of the correct use of these products. Products containing more than 12.8 mg codeine are available on prescription only.[46]

The Maldives takes an infamously strict line on medicines, with many common drugs, notably anything with containing codeine being banned unless you have a notarized and authenticated doctor's prescription. Visitors breaking the rules, even inadvertently, have been deported or imprisoned.

Codeine is not allowed without a medical prescription. Codeine is sold under the name Farmacod and its concentration does not exceed 15 mg. There is a known combination of acetylsalicylic acid, paracetamol and codeine phosphate hemihydrate named Aspaco that is allowed without a medical prescription but its case is signed with an exclamation red symbol which means that driving won't be allowed during treatment. There are no sanctions whether the drug is given without a prescription.

According to ITAR-Tass and Austria Presse-Agentur, OTC availability of codeine products was rescinded nationwide in 2012 because of the discovery of the Krokodil method of underground desomorphine synthesis. Opponents of the move point out that codeine has not been available OTC in 22 of Russia's oblasts for years and the demand will call forth its own supply, meaning that only legitimate end users are negatively impacted (activist quoted in Pravda story on issue)

As of 2 January 2014 patients will be required to supply key personal details including ID numbers, when they purchase a range of popular over-the-counter medications containing codeine in South Africa. By this date the Misused Substance Database, a central database of all purchases of medication containing the active ingredient, will be operational, and codeine is the first substance to be captured and controlled in this project. In time, other commonly misused ingredients, such as those used in sleeping tablets, will be included in this process.[47]

Codeine tablets or preparations require a prescription in Spain, although this is often not enforced and many pharmacies will sell codeine products without the requirement of a prescription.[citation needed]

Codeine preparations are available as over the counter pharmacy medicines in Sri Lanka. The most common preparation is Panadeine, which contains 500 mg of Paracetamol and 8 mg of Codeine. But cough syrup containing Codeine is banned, even with a prescription.

The UAE takes an exceptionally strict line on medicines, with many common drugs, notably anything containing codeine being banned unless you have a notarized and authenticated doctor's prescription. Visitors breaking the rules, even inadvertently, have been deported or imprisoned. The US Embassy to the UAE maintains an unofficial list of what may not be imported.

In the United Kingdom, the sale and possession of codeine are restricted separately under law.

Neat codeine and higher-strength codeine formulations are generally prescription-only medicines (POM) meaning that the sale of such products is restricted under the Medicines Act 1968. Lower-strength products containing combinations of up to 12.8 mg of codeine per dosage unit, combined with paracetamol, ibuprofen or aspirin are available over the counter at pharmacies. Codeine linctus of 15 mg per 5 ml is also available at some pharmacies, although a purchaser would have to request it specifically from the pharmacist.

Under the Misuse of Drugs Act 1971 codeine is a Class B controlled substance or a Class A drug when prepared for injection.[48] The possession of controlled substances without a prescription is a criminal offence.[49] However, certain preparations of codeine are exempt from this restriction under Schedule 5 of the Misuse of Drugs Regulations 2001. It is thus legal to possess codeine without a prescription, provided that it is compounded with at least one other active or inactive ingredient and that the dosage of each tablet, capsule, etc. does not exceed 100 mg or 2.5% concentration in the case of liquid preparations. The exemptions do not to apply to any preparation of codeine designed for injection.[50]

Bulk codeine falls into two categories, that for mixing into medicines which can be Schedule II, III, IV, or V depending on concentration, and that used as a precursor in making other drugs. Both categories of bulk codeine are Schedule II controlled substances, both with an ACSCN of 9050: bulk codeine for sale has an aggregate annual manufacturing quota for the US of 49 506.25 kilos as of 2013. Bulk codeine for conversion has a quota of 81 250 kilos, both unchanged from the prior year.

The narcotic content number in the US names of codeine tablets and combination products (i.e., Tylenol With Codeine No. 3, Emprin With Codeine No. 4, the former Emprazil With Codeine No. 5 and pure codeine tablets) are as follows: No. 1 - 7½ or 8 mg (1/8 grain), No. 2 - 15 or 16 mg (1/4 grain), No. 3 - 30 or 32 mg (1/2 grain), No. 4 - 60 or 64 mg (1 grain), No. 5 - 90 or 96 mg (1 1/2 grains). The Canadian "Frosst 222"[51] series is identical to the above list: "222" contains 8 mg codeine, "282" 15 mg, "292" 30 mg, and "293" 60 mg. This system, which is also used at present in the trade names of some dihydrocodeine and ethylmorphine products both in and outside of North America, was inaugurated with the Pure Food and Drug Act of 1906 and related legislation and refined since. For example, the dihydrocodeine/aspirin/caffeine capsule Synalgos DC and its generic equivalents and paracetamol analogues are labelled as No. 2 (16 mg) or No. 3 (32 mg) in most cases; the No. 1 and 4 products disappeared decades ago, as did plain Synalgos without narcotic content.

Equivalent scales for labeling stronger opioids such as diacetylmorphine (heroin), morphine, opium salts mixtures, and others were in common use in the past, and on occasion one can find past references to brand names for hydrocodone (invented 1920, introduced in US 1943), hydromorphone (invented 1924), oxycodone (invented 1916), paregoric and similar drugs containing narcotic content numbers. For example. from circa 1900 to 1925, the most common cough medicine was terpin hydrate With Heroin Elixir No. 2.

Contrary to the advertising matter of some pharmacies, 60 mg is No. 4, not No. 6, and tablets with 45 mg of codeine are not No. 4 and would in all likelihood be classified as No. 3½ under that system. Whether the scale goes to No. 6 (presumably 2 grains or 120 or 128 mg) and higher is moot at this point, as in the United States and Canada single-dose-unit concentrations of more than 64 mg are no longer manufactured. The United States Controlled Substances Act of 1970 does place dosage unit strengths of 90 mg of codeine and higher in Schedule II, even if mixed with another active ingredient.

Oral tablets, hypodermic tablets, liquid forms, and capsules of less common doses are available in some cases the equivalent dihydrocodeine, dionine, benzylmorphine, and opium dosages were previously available in North America (and in most cases still are in other countries, particularly the 45 mg paracetamol/codeine and 50 and 100 mg single-ingredient codeine tablets).

In the United States, codeine is regulated by the Controlled Substances Act. Federal law dictates that codeine be a Schedule II controlled substance when used in products for pain-relief that contain codeine alone or more than 90 mg per dosage unit. Tablets of codeine in combination with aspirin or paracetamol (paracetamol/Tylenol) made for pain relief are listed as Schedule III; and cough syrups are Schedule III or V, depending on formula. The paracetamol/codeine pain-relief elixir (trade name Tylenol Elixir with Codeine) is a Schedule IV controlled substance.[52]

Some states have chosen to classify Schedule V codeine preparations into a more restrictive schedule in order to cut down the abuse of prescription codeine preparations. Minnesota, for instance, has chosen to reclassify Schedule V codeine preparations (such as Cheratussin) as a Schedule II controlled substance.[53]

Preparations for cough or diarrhea containing small amounts of codeine in combination with two or more other active ingredients are Schedule V in the US, and in some states may be dispensed in amounts up to 4 fl. oz. per 48 hours (one or two states set the limit at 4 fl. oz. per 72 hours) without a prescription. Schedule V specifically consigns the product to state and local regulation beyond certain required record-keeping requirements (a dispensary log must be maintained for two years in a ledger from which pages cannot easily be removed and/or are pre-numbered, and the pharmacist must ask for photo identification) and also maintain controlled substances in the closed system at the root of the régime intended by the Controlled Substances Act of 1970; the codeine in these products was a Schedule II substance when the company making the Schedule V product acquired it for mixing up the end-product.

In locales where dilute codeine preparations are non-prescription, anywhere from very few to perhaps a moderate percentage of pharmacists will sell these preparations without a prescription. However, many states have their own laws that do require a prescription for Schedule V drugs. The December 2008 issue of The Bulletin of the National Codeine OTC Lobby (Vol. XVIII, No. 4) listed 12 states with some kind of OTC access to codeine, noting that small independent pharmacies are the most likely to have it. This situation is roughly equivalent to that in February 1991, when the aforementioned organisation undertook its first comprehensive study of Schedule V and overall codeine, dihydrocodeine, ethylmorphine, and hydrocodone availability.

Other drugs that are present in Schedule V narcotic preparations like the codeine syrups are ethylmorphine and dihydrocodeine. Paregoric and hydrocodone were transferred to Schedule III from Schedule V even if the preparation contains two or more other active ingredients, and diphenoxylate is usually covered by state prescription laws even though this relative of pethidine is a Schedule V substance when adulterated with atropine to prevent abuse.

Codeine differs from morphine in that the hydroxyl group on C3 of the aromatic ring is methylated. The content of codeine in opium does not satisfy medical requirements and therefore codeine is made in a semisynthetic manner from morphine by selective methylation of the aromatic hydroxyl group on C3.

The usual methylating agents result in the methylation of both hydroxyl groups.

Selective methylation of the hydroxyl group at C3 of the aromatic ring can be accomplished using diazomethane, nitrosomethylurethane, or nitrosomethylurea. However, use of these reagents presents certain difficulties in completing the reaction on large industrial scale. It was suggested that trimethylphenylammonium chloride or dimethylaniline methyltoluenesulfonate in the presence of sodium alkoxides could be used as methylating agents.